The present invention relates to a microwave heating apparatus such as an electronic oven. More particularly, it relates to an improvement in the shape of a waveguide having a plurality of power feeding ports.
There have been proposed microwave heating apparatuses with a waveguide in which a single power feeding port is formed. FIGS. 31 and 32 are respectively longitudinal cross-sectional views in schematic forms of an electronic oven which utilizes a conventional microwave heating apparatus. Description will be made with reference to FIGS. 31 and 32. A reference numeral 1 designates a heating chamber, a numeral 2 designates a turn table positioned at the lower portion of the heating chamber 1 and adapted to receive thereon a material to be heated 6, a numeral 3 designates a magnetron for generating microwaves, a numeral 7 designates an antenna for emitting the microwaves, and a numeral 4 designates a waveguide which is attached to the outer wall of the ceiling of the heating chamber 1 and guides the microwaves emitted from the magnetron 3 to a power feeding port 5 formed in the ceiling of the heating chamber 1.
The conventional electronic oven as shown in FIG. 31 is so adapted that the material to be heated 6 (hereinbelow, referred to as a heating material) is placed on the turn table 2, and when a door is closed to actuate a power switch (not shown), the turn table 2 is started to rotate, and at the same time, microwaves are emitted from the antenna 7 of the magnetron 3. The microwaves are supplied to the heating chamber 1 via the waveguide 4 and the power feeding port 5 to thereby heat the heating material 6.
FIG. 32 shows another conventional technique, wherein a power feeding port 5 is formed at the upper portion of a side wall of the heating chamber 1. Microwaves are supplied into the heating chamber 1 through the power feeding port 5. A waveguide 4 attached to the outer surface of the side wall of the heating chamber 1. The function of the apparatus as shown in FIG. 32 is the same as that in FIG. 31.
In the conventional microwave heating apparatuses, since only one power feeding port 5 for supplying the microwaves into the heating chamber 1 is formed in the ceiling or a side wall, the microwaves can not be supplied uniformly to the heating material 6 as shown in FIGS. 31 and 32 to thereby often cause uneven heating to the heating material. Further, it takes much time to heat the heating material 6 depending on the position of the heating material, and much power is consumed.
To eliminate the above-mentioned problems, microwave heating apparatuses having a plurality of power feeding ports formed in a waveguide are proposed as shown in FIGS. 33 and 34. FIG. 33 is a longitudinal crosssectional view of a conventional microwave heating apparatus having power feeding ports formed in the ceiling of the heating chamber, which is described in, for instance, Japanese Examined Utility Model Publication 15589/1986. FIG. 34 is a longitudinal cross-sectional view of a conventional microwave heating apparatus having power feeding ports in a side surface of the heating chamber. In FIG. 33, a numeral 1 designates a heating chamber, a numeral 8 designates a waveguide attached to the top surface of the heating chamber 1 so that an end of the waveguide projects from the right side portion of the top surface, a numeral 9 designates three power feeding ports formed in the top surface of the heating chamber 1 so as to communicate the waveguide 8 with the heating chamber 1, a numeral 3 designates a magnetron as a microwave oscillating apparatus which is connected to the lower end of the projecting portion of the waveguide 8 and has an antenna 7 extending in the waveguide 8. In FIG. 34, a waveguide 10 is provided at a side surface of the heating chamber 1, and two power feeding ports are formed at the side surface of the heating chamber 1 so as to communicate the waveguide 10 with the heating chamber 1.
Let's assume that a wavelength of radiowaves oscillated from the antenna 7 is .lambda.g. The distance from the center of the antenna 7 to the surface of the waveguide 8 facing oppositely the surface where the power feeding ports are formed is called a back plunger which has a wavelength of .lambda.g/4, and it is usually determined to be 18.6 mm-22 mm.
With respect to the backup plunger, it is introduced in, for instance, a publication "A lecture of practical microwave, a microwave circuit", p.148-149 by Mrs. Kunihiro Suetake and Shuichi Hayashi published by Ohm Sha on Oct. 31, 1958, as follows. "Normally, a short circuit plate S is provided at a position apart from a length of about 1/4 from an antenna to cause a short circuit as shown in Figures (FIGS. 35 and 36). Thus, there is obtainable radiowaves propagating in the opposite direction".
The microwave heating apparatus having more than two power feeding ports with the back plunger is disclosed in addition to the above-mentioned conventional apparatus, in publications such as Japanese Examined Patent Publication 37504/1987, Japanese Examined Patent Publication 30077/1985, Japanese Examined Utility Model Publication 22080/1987, Japanese Examined Utility Model Publication 31999/1987, Japanese Examined Utility Model Publication 30798/1987, Japanese Examined Utility Model Publication 11916/1986, Japanese Examined Utility Model Publication 35988/1985, Japanese Examined Utility Model Publication 35991/1985 and so on.
In operations, microwaves oscillated from the antenna 7 of the magnetron 3 are propagated toward the heating chamber 1 by the back plunger provided in the waveguide 8 or 10 and are introduced into the heating chamber 1 through the power feeding ports 6, whereby a material to be cooked placed in the heating chamber 1 is heated.
In the conventional microwave heating apparatus having the above-mentioned construction, the shape of the waveguide 8 or 10 was inevitably complicated by satisfying both requirements that the back plunger has to be provided and a plurality of power feeding ports 9 or 11 have to be provided at desired positions. Namely, in a case that the power feeding ports 9 are formed at the central portion of the top surface of the heating chamber 1 and both end portions with respect to the central portion in order to reduce uneven heating as shown in FIG. 33, the back plunger is assured by projecting an end of the waveguide 8 from an end of the heating chamber, by connecting the magnetron 3 at the lower portion of the projecting part, and by inserting the antenna 7.
In a case that the power feeding ports are formed at the upper and lower portions of a side surface of the heating chamber 1 as shown in FIG. 34, the back plunger is assured by projecting the central portion of the waveguide 10 in the lateral direction, by connecting the magnetron 3 to the lower portion of the projecting part, and by inserting the antenna 7. In either case of the waveguides 8, 9 as shown in FIGS. 33 and 34, the magnetron 3 and its connecting portion were unevenly projected with respect to the heating chamber 1. Accordingly, there arose problems that the structure of the waveguide 8 or 10 was complicated, the number of machining steps was increased, hence, the manufacturing cost became high, a freedom in determining the position of structural elements was limited, and the size of a microwave heating apparatus main body became large.
Definitions of the uneven projection and the even projection will be described. In a case of a waveguide 8 projecting from the top surface of a microwave heating apparatus main body as shown in FIG. 37, a projecting portion is called an uneven projection 12. On the other hand, in a case of a waveguide 8 whose projecting portion 13 extend on and along another surface as shown in FIG. 38, a projecting portion is called uneven projection 13. There is proposed another technique as shown in FIG. 39 that a waveguide 8 is attached to the top surface of the heating chamber so as to extend from an end to the other end, but so as not to project from the heating chamber 1, power feeding ports 14 are formed at the central portion and an end portion so as to communicate the waveguide 8 with the heating chamber 1, and an antenna 7 for a magnetron 3 is arranged at a position apart from the end of the waveguide 8 by a wavelength of .lambda.g/4. In this technique, however, the length of the waveguide 8 is inevitably large and it is impossible to form the power feeding ports at both end portions. Further, it is difficult to reduce uneven heating because the plural power feeding ports are formed.